1. Field of the Invention
The present invention disclosed in the specification relates to a thin film transistor and a method of making thereof. Further, the present invention relates to a method of making a display device of an active matrix type utilizing thin film transistors.
2. Description of Related Art
A thin film transistor has been known as a device for constituting an active matrix type liquid crystal display device. Particular attention is being paid to technologies utilizing a thin film transistor using a silicon thin film having crystallinity.
A thin film transistor using a silicon film having crystallinity is characterized in that a high speed operation is feasible and a CMOS (Complementary Metal Oxide Semiconductor) circuit can be constituted.
When a thin film transistor using a silicon thin film having crystallinity is used, an active matrix circuit and peripheral drive circuits for driving the active matrix circuit can be integrated on one sheet of a glass substrate (or quartz substrate) by making use of such a characteristic.
However, a crystalline silicon thin film provided in the current technology is not in a single crystal state but in a polycrystal state or a microcrystal state. Defects or impurities are included in such a film (referred to as crystalline silicon film) at a comparatively high level. Accordingly, there poses a problem where the structure of a bonded portion of different conductive materials is electrically inferior. The problem gives rise to a factor causing an aging change (generally amountable to deterioration) of an OFF current or properties in the operation of a thin film transistor.
For example, the problem where the OFF current (a current made to flow between a source and a drain in the OFF operation) is comparatively large is a problem common to thin film transistors of a P-channel and an N-channel type. Furthermore, the mobility of carriers in the P-channel type thin film transistor is lower than that of carriers of the N-channel type one. Also, there poses a problem in the N-channel type transistor where deterioration by hot carriers (deterioration thereof particularly at a bonded portion) is significant.
It is preferable to attain simultaneous resolution of the above-described problems when an integrated circuit using thin film transistors are constituted. Especially, when P-channel type and N-channel type transistors are simultaneously formed (separately formed) on a same substrate, a difference in characteristics between the P-channel type transistors and the N-channel type ones needs to correct.
The reason is that a circuit, (generally constituted based on a CMOS circuit) having excellent characteristics cannot be obtained when only the N-channel transistor or the P-channel transistor is provided with a low OFF current characteristic or is provided with a high mobility.
It is a problem of the present invention disclosed in the specification to provide a thin film transistor having a low OFF current value. Also, it is a problem thereof to provide thin film transistors of a P-channel type and an N-channel type where the difference in characteristics is corrected.